B.Sc. 2nd YEAR I.D.
STRUCTURES AND BUILDINGS
In engineering and architecture, a structure is a body or assemblage of bodies
in space to form a system capable of supporting loads. Physical structures
include man-made and natural arrangements. Buildings, aircraft, soap films,
skeletons, anthills, beaver dams and salt domes are all examples of physical
structures. The effects of loads on physical structures are determined
through structural analysis. Structural engineering refers to engineering of
Built structures are a subset of physical structures resulting from construction.
These are divided into buildings and non-building structures, and make up
the infrastructure of a human society. Built structures are composed
of structural elements such as columns, beams and trusses. Built structures are
broadly divided by their varying design approaches and standards,
into categories including Building structures, Architectural structures, Civil
engineering structures and Mechanical structures.
FORCE AND LOAD
The influence of the environment on structures takes
the form, principally, of Loads and Forces. Here the
word 'environment' is taken to mean anything in
contact with the structure (e.g. vehicles, furniture,
people etc.), including the structure itself. Such
primary environmental influences as wind,
temperature, and earthquake affect the structure by
exerting forces on it.
FORCE AND LOAD
Influence on a body, causing (or attempting to cause) the Movement of the body or part of it,
or causing a change in its movement, if it is already in motion. This is the common definition of
force encountered in the literature. It is interesting to note that even though force is one of the
most fundamental concepts in physics, its definition is indirect, relying on its effect. This is an
indication of the complexity of this concept and the difficulty in visualising it. The definition of
force through the concept of motion, a concept which is easy to grasp intuitively, enables easy
visualisation of forces, and brings forth the extremely important relationship between force
and motion. This, in fact, is the source of the force-shape relationship which is the focal point
of this work. A force is a Vector. A vector is a parameter (a physical quantity) characterised by a
magnitude, (or 'intensity') and a direction. Relying on the correlation between force and
motion, it is convenient to visualise a vector in terms of motion: when an object moves from
point A to point B, the magnitude of the distance travelled is not enough to define the position
of point B, relative to A. We need to know the direction as well. Distance, like force, is a vector.
A vector is described graphically as an arrow, pointing in the direction of the vector and having
a length representative of the magnitude.
UNITS OF MEASUREMENT
Force, like distance, is one of the fundamental physical entities, measured in
one of the basic units. The basic force unit is the Newton (denoted N) and
its multiples -kilo-Newton (kN, one thousand Newtons) and Mega-Newton
(MN, one million Newtons). As a rule, the international system of units is
used throughout this text, with some exceptions. This system employs the
Newton (N) and its derivatives for force units, and the metre (m) or
millimetre (mm) as length units. Centimetre (cm) is also used occasionally.
A force applied to a structure by the
environment or by any object (including the
structure itself or other structures). Alternative
definition: any External force applied to the
structure, other than a Reaction force.
TYPES OF LOAD
The structures in question are buildings, bridges, monuments, signposts etc. There
are two major types of loads: Gravity loads, which are usually vertical, and
Environmental loads, which are often horizontal (e.g. earthquake) but can generally
take any direction. Although all loads were defined as arising from the influence of
the environment, the term Environmental load refers to a subclass of loads defined
TYPES OF GRAVITY LOAD:
Gravity loads are the effect of the weight of objects on the structure, including the
weight of the
structure itself (weight is a force). Two kinds are distinguished:
Dead load: Load resulting from the self weight (SW) of the structure and of any
permanently attached components, such as walls, flooring, permanent partitions etc.
Live load: Load arising from the function of the structure, including attached
components whose location is not fixed, such as movable partitions. Live loads are a
result of the weight of the loading objects (vehicles, furniture, goods, people etc.)
and are mostly vertical (snow load is also considered live load). In some cases,
however, loads may be applied in non-vertical directions, for instance loads due to
braking of vehicles, loads transmitted
through pulleys, earth or hydrostatic pressure etc.
TYPES OF GRAVITY LOAD
Environmental loads are not a direct result of the weight of objects, but of
movement in the structure's environment. The most common environmental loads
are Wind load and Earthquake load. Wind load is a result of moving air tilting the
structure. Earthquake load is a result if the movement of the earth in which the
structure is founded. The force-movement relation is reciprocal. In the same way
that force causes movement, force can be caused by movement. In the above
instances, the movement (of the air or the ground) causes forces on the structure
and these forces, in turn, cause movement of the structure and of parts of the
structure relative to one another.
OTHER ENVIRONMENTAL INFLUENCES
Other environmental influences are movements which may cause
Internal forces in certain structures. In other cases they only cause
These influences include temperature effects -change of temperature
or temperature difference over parts of the structure, e.g. between
the inside and the outside; Support settlement settlement (sinking) of foundations by differing amounts; and so on.
Some other influences affecting dimensions of components of the
structure are also considered environmental effects because of the
similarity to the influence of temperature and settlement. These
include statistical variation in component dimensions ('lack of fit'),
and deliberately induced Deformations.
Load has been described in general terms, as the
overall force acting on the structure, causing
movement in it. In practice, a load applied to a
structure is distributed, or 'spread', over its surface in
certain ways, for instance snow over the roof surface,
vehicles over a bridge deck etc. A load distributed
over a portion of the structure is termed Distributed
Two major types of load distribution are
UNIFORMLY DISTRIBUTED LOAD
A uniformly distributed load is a load that has
been spread over a beam in such a way that
each unit of length has equal weight. The
centre of gravity of such a load is at the point
where it balances when placed on a knife
CONCENTRATED OR POINT LOAD
A point load refers to a point where a bearing or
structural weight is intense and transferred to the
foundation. A good example is when a hammer
applies a force at one point and regardless of what
point the hammer's head hits, force is applied.
VALUES OF LOAD FOR DESIGN PURPOSE
Load values are specified in Codes or
Standards. Codes and standards are design aids
as mentioned earlier.
The ability of the structure and every part of it to support the load without
collapsing, taking into account uncertainties in the values of actual loads
and in the strength and behaviour of the structure.
The ability of the structure to ensure its satisfactory functioning. This
implies particularly limitations on the magnitude of movements under
various applied loads (Deflection, vibration etc.).
Movement is the result of the action of force, or a
combination of forces. In general, movement can
include such parameters as distance, speed, time,
acceleration. In the context of this work only the
distance is of interest, both in its own right and as an
indicator of the force causing it.
Displacement is the distance through which a body, or
a point on the body, moves as a result of the action of
force. This distance is a vector. It is characterised by a
magnitude -the amount of travel -and a direction.
Rotation is a kind of movement (displacement) but it is more
complex than the linear movement implied so far. When an
object rotates there is a point in it which does not move at all
and different points on it have different displacements –
different magnitudes and directions of distance.
FORCE COUPLE AND MOMENT
A rotation cannot be affected by a single force vector of the type we have encountered. Since
the body as a whole does not move, there can be no net force acting on it (see force Resultant
below). We can imagine a rotation of a body if the body is acted upon by two forces of equal
magnitude (say P) and opposite direction, such that the lines of action of the two forces are
offset by a certain distance (a, say). Such a pair of forces is termed a Force couple, or Couple for
short. The body as a whole cannot move, because the two forces act in opposite directions. But
at each of the two points of application of forces, the corresponding force moves the point in
its direction. The result is that the two points move in opposite directions, causing the rotation
of the body.
Moment is defined as the perpendicular distance from a point to a line or a surface, and is derived from the
mathematical concept of moments . It is frequently used in combination with other physical quantities as
in moment of inertia, moment of force, moment of momentum, magnetic moment and so on.
Moment is also used colloquially for different physical quantities that depend upon distance. For example, in
engineering and kinesiology the term moment is often used instead of the more complete term moment of
force. A moment of force being the product of the distance of a force from an axis times the magnitude of the
force, i.e., F × d, where F is the magnitude of the force and d is the moment of the force. See torque for a
more complete description of moments of force or couple for the related concept free moment of force also
known as a force couple.
It may also be used when the distance is squared, as in moment of inertia. The moment of inertia is the
"second moment" of mass of a physical object. This is the object's resistance or inertia to changes in its
angular motion. It is roughly the sum of the squared distances (i.e., moments) of the object's mass particles
about a particular axis .
Normally a structure is not subjected to a single force, but to .a
combination of several loads and other forces, in different
directions and locations. In order to understand how the
structure responds to such load combinations, it is necessary to
know how to handle such combinations -how to operate with
SUMMALATION OF VECTORSRESULTANT
When a number of forces (or any vectors) act on an object
simultaneously, the Resultant force (or Resultant vector) is a
single force (vector) which, if acting alone on the object would
have the same effect as the combined forces (vectors). It is said
to represent the sum of the vectors, or the Vectorial sum. It is
easy to visualise a resultant vector and a way to derive it if we
think of displacements rather than forces. If we think of each
vector as a corresponding displacement, and instead of
applying them simultaneously apply them sequentially (the
final result being the same), then the resultant displacement is
the distance from the starting point to the final point. To obtain
the resultant graphically, plot the individual vectors tail to
head. The resultant is the vector joining the tail of the first
vector with the head of the last.
The magnitude of the resultant of a set of parallel forces is simply the sum
of the forces and the direction is parallel with the forces. The question is the
location of the resultant relative to a reference point.
To obtain the location of the resultant force, apply at the reference point
imaginary forces of equal magnitude and opposite sense to the given forces.
These imaginary forces form couples with the original forces. Their sum
forms a couple with the resultant force.
The location of the resultant force is determined from the condition that its
moment is equal to the sum of the moments of the given forces. This is
because the effect of the resultant has to be the same as that of the given
forces in every respect, including rotation with respect to any point.
GENERAL SYSTEM OF FORCES
This expression can be used to obtain the location of the origin
(the application point) of the resultant of any set of forces (not
necessarily parallel), by working with their components. Each
force is replaced by its components, having the same point of
application as the force.
The components parallel to any axis (x , y) form a set of parallel
forces and so the expression above gives the location of the
component of the resultant parallel to the same axis (i.e. its
distance from the axis). The origin of the resultant is at the
intersection of the directions of the two components.
NUMBER v/s QUANTITY
SI BASE UNITS
UNITS IN WHICH MEASUREMENTS ARE TAKEN
1 FEET(ft)=12 in
1 YARD(yd)=3 ft
1 MILE(mi)=5,280 ft
1,760 YARDS (yd)